migrate_pages: move THP/hugetlb migration support check to simplify code

This is a code cleanup patch, no functionality change is expected.  After
the change, the line number reduces especially in the long
migrate_pages_batch().

Link: https://lkml.kernel.org/r/20230213123444.155149-10-ying.huang@intel.comSigned-off-by: "Huang, Ying" <ying.huang@intel.com>
Suggested-by: Alistair Popple <apopple@nvidia.com>
Reviewed-by: Zi Yan <ziy@nvidia.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Oscar Salvador <osalvador@suse.de>
Cc: Matthew Wilcox <willy@infradead.org>
Cc: Bharata B Rao <bharata@amd.com>
Cc: Xin Hao <xhao@linux.alibaba.com>
Cc: Minchan Kim <minchan@kernel.org>
Cc: Mike Kravetz <mike.kravetz@oracle.com>
Cc: Hyeonggon Yoo <42.hyeyoo@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

mm/migrate.c

@@ -1117,9 +1117,6 @@ static int migrate_folio_unmap(new_page_t get_new_page, free_page_t put_new_page
 	bool locked = false;
 	bool dst_locked = false;
 
-	if (!thp_migration_supported() && folio_test_transhuge(src))
-		return -ENOSYS;
-
 	if (folio_ref_count(src) == 1) {
 		/* Folio was freed from under us. So we are done. */
 		folio_clear_active(src);
@@ -1380,16 +1377,6 @@ static int unmap_and_move_huge_page(new_page_t get_new_page,
 	struct anon_vma *anon_vma = NULL;
 	struct address_space *mapping = NULL;
 
-	/*
-	 * Migratability of hugepages depends on architectures and their size.
-	 * This check is necessary because some callers of hugepage migration
-	 * like soft offline and memory hotremove don't walk through page
-	 * tables or check whether the hugepage is pmd-based or not before
-	 * kicking migration.
-	 */
-	if (!hugepage_migration_supported(page_hstate(hpage)))
-		return -ENOSYS;
-
 	if (folio_ref_count(src) == 1) {
 		/* page was freed from under us. So we are done. */
 		folio_putback_active_hugetlb(src);
@@ -1556,6 +1543,20 @@ static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
 
 			cond_resched();
 
+			/*
+			 * Migratability of hugepages depends on architectures and
+			 * their size.  This check is necessary because some callers
+			 * of hugepage migration like soft offline and memory
+			 * hotremove don't walk through page tables or check whether
+			 * the hugepage is pmd-based or not before kicking migration.
+			 */
+			if (!hugepage_migration_supported(folio_hstate(folio))) {
+				nr_failed++;
+				stats->nr_failed_pages += nr_pages;
+				list_move_tail(&folio->lru, ret_folios);
+				continue;
+			}
+
 			rc = unmap_and_move_huge_page(get_new_page,
 						      put_new_page, private,
 						      &folio->page, pass > 2, mode,
@@ -1565,16 +1566,9 @@ static int migrate_hugetlbs(struct list_head *from, new_page_t get_new_page,
 			 *	Success: hugetlb folio will be put back
 			 *	-EAGAIN: stay on the from list
 			 *	-ENOMEM: stay on the from list
-			 *	-ENOSYS: stay on the from list
 			 *	Other errno: put on ret_folios list
 			 */
 			switch(rc) {
-			case -ENOSYS:
-				/* Hugetlb migration is unsupported */
-				nr_failed++;
-				stats->nr_failed_pages += nr_pages;
-				list_move_tail(&folio->lru, ret_folios);
-				break;
 			case -ENOMEM:
 				/*
 				 * When memory is low, don't bother to try to migrate
@@ -1664,6 +1658,28 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 
 			cond_resched();
 
+			/*
+			 * Large folio migration might be unsupported or
+			 * the allocation might be failed so we should retry
+			 * on the same folio with the large folio split
+			 * to normal folios.
+			 *
+			 * Split folios are put in split_folios, and
+			 * we will migrate them after the rest of the
+			 * list is processed.
+			 */
+			if (!thp_migration_supported() && is_thp) {
+				nr_large_failed++;
+				stats->nr_thp_failed++;
+				if (!try_split_folio(folio, &split_folios)) {
+					stats->nr_thp_split++;
+					continue;
+				}
+				stats->nr_failed_pages += nr_pages;
+				list_move_tail(&folio->lru, ret_folios);
+				continue;
+			}
+
 			rc = migrate_folio_unmap(get_new_page, put_new_page, private,
 						 folio, &dst, pass > 2, avoid_force_lock,
 						 mode, reason, ret_folios);
@@ -1675,36 +1691,9 @@ static int migrate_pages_batch(struct list_head *from, new_page_t get_new_page,
 			 *	-EAGAIN: stay on the from list
 			 *	-EDEADLOCK: stay on the from list
 			 *	-ENOMEM: stay on the from list
-			 *	-ENOSYS: stay on the from list
 			 *	Other errno: put on ret_folios list
 			 */
 			switch(rc) {
-			/*
-			 * Large folio migration might be unsupported or
-			 * the allocation could've failed so we should retry
-			 * on the same folio with the large folio split
-			 * to normal folios.
-			 *
-			 * Split folios are put in split_folios, and
-			 * we will migrate them after the rest of the
-			 * list is processed.
-			 */
-			case -ENOSYS:
-				/* Large folio migration is unsupported */
-				if (is_large) {
-					nr_large_failed++;
-					stats->nr_thp_failed += is_thp;
-					if (!try_split_folio(folio, &split_folios)) {
-						stats->nr_thp_split += is_thp;
-						break;
-					}
-				} else if (!no_split_folio_counting) {
-					nr_failed++;
-				}
-
-				stats->nr_failed_pages += nr_pages;
-				list_move_tail(&folio->lru, ret_folios);
-				break;
 			case -ENOMEM:
 				/*
 				 * When memory is low, don't bother to try to migrate